144 
THE CAMBRIAN VOLCANOES 
BOOK III 
Andrew Kamsay and his colleagues on the Geological Smwey that on the 
mainland of Wales no base is ever found to the Cambrian system. More 
recently certain conglomerates have been fixed upon as the true Cambrian 
base, both in South and North Wales, and endeavours have been made to 
trace an uneonformability at that line, all rocks below it being treated as 
pre-Cambrian. But conglomerates do not necessarily mark a stratigraphical 
discordance, and in South Wales there is no trace of any uneonformability 
lietween the strata above and below the supposed line of break.^ Professor 
Bonney has shown that in North Wales several zones of conglomerate have 
been erroneously identified as the supposed basal platform of the Cambrian 
series, and more recently Mr. Blake has pointed out that some of these 
conglomerates are unquestionably Lower Silurian. 
My own examination so far confirms the conclusions arrived at by these 
observers. Like my predecessors in the Geological Survey, however, I have 
been unable to detect anywhere in Caernarvonshire or Merionethshire a base 
to the Cambrian system, and I am compelled to agree with them in 
regarding as Cambrian (partly even as Lower Silurian) all the rocks from 
Bangor to Llanllyfni, which have more recently been classed as pre-Cambrian. 
But though thus supporting their general stratigraphy, I am bound to 
acknowledge that they failed to recognize the existence of a great volcanic 
series below the Arenig horizon. The existence of this series, noticed by 
Sedgwick, was first definitely stated by Professor Hughes,'^ and his statements 
have been confirmed and extended by subsequent observers, notably by 
Professor Bonney and Mr. Blake. The Cambrian period is thus proved to 
have been perhaps even more continuously volcanic than the Lower Silurian 
period was in Wales. 
The following table shows the subdivisions of the Cambrian system now 
recognized in Britain : — 
Wales. 
(Ranging up to 12,000 feet or more.) 
Western England. 
(About 3000 feet.) 
Upper or ( Tremadoc Slates 
Olemts 1 Lingula Flags {Lin- 
Zones. ( ijulcUa, Olenus, etc.). 
Shinetou Shales {Dictyograptus 
or Dictyoncma, Olenus, etc.). 
Middle i 
or Para- 1 Meneviau group 
doxwlcs f iPa.radovides). 
Zones. J 
Conglomerates and limestones 
(Comley), with Paradoxides, 
etc. 
( Harlech and Llan- 
Low'er or beris group rvith 
Olmdlus\ basement volcanic 
Zones. rocks ; bottom not 
f seen. 
Thin quartzite passing up 
into gi’een flags, grits, shales 
and sandstone (Comley Sand- 
stone), containing Ole^iellus. 
X.W. Scotland. 
(About 2000 feet.) 
Limestone."!, about 1500 feet 
thick, divisible into seven 
groups {ArehccocycUlms, 
Madurea, OphiUta, Murehi- 
sonia, Orthoeeras and vast 
quantities of annelid cast- 
ings). 
Shales (“fucoid beds”), with 
Olenellua, Salterdla, etc. 
Quartzites with annelid bur- 
rows. The base of the series 
lies unoonfomiably on pre- 
Cambrian rocks. 
1 See a discussion of this subject in Quart. Journ. Geol. Soc. vol. xxxix. (1883), p. 305. 
2 Froc. Camh. Phil. Soc. vol. iii. (1877), p. 89. The Cambrian volcanic areas of North Wales 
are represented in Map II. 
